This invention relates to geothermal wellhead apparatus and in particular to apparatus for controlling the flow of fluids and debris from a geothermal well.
The flow control of fluids from a well during drilling operations or production is complicated by the debris produced with the fluids. Debris, such as formation solids, pieces of well casing or well hardware, and even scale or corrosion products, impact with sufficient force against the interior of the wellhead apparatus and downstream equipment to cause excessive wear. Solids produced at a high velocity tend to erode valve seats, pipe walls, and other materials they contact. In addition, solids of appreciable size can become lodged in orifices or valve seats and thereby restrict fluid flow. In some cases a throttling valve, orifice or choke bean can be rendered totally useless due to a combination of wear and plugging. Debris can also accumulate in many low points in the flowline, causing flow restrictions and influencing corrosion and erosion rates.
Flow of fluids from geothermal steam wells is particularly subject to the problem of entrained debris moving at high velocity. The high velocity of geothermal flow, which ranges up to a million pounds per hour of produced fluids from a single well, entrains more and larger solids in geothermal fluids than are found in flow from other wells. For instance, as much as 100 pounds per month of debris can issue from a typical steam-producing well.
The problems associated with entrained debris from geothermal steam wells are particularly acute during air-drilling operations when compressed air is used to circulate rock cuttings out of the wellbore. As zones are penetrated that contain compressed geothermal fluids, large volumes of steam are suddenly released which accelerate the rock cuttings to very high velocities. Due to the high velocity of the fluids, large pieces of abrasive rock are readily entrained and carried out of the reservoir. The problem is complicated by the necessity to deflect the upward vertical flow out of the wellbore towards the horizontal at the surface to send it to a rock separator and muffler. The interior surface of the equipment used to turn the fluid flow, commonly called the banjo box, can be worn completely through during drilling of a single geothermal well, with the result that drilling crews are exposed to injury from scalding steam and sharp-edged rock cuttings travelling at high velocity. In addition, replacing the defective banjo box requires costly shutdown of the entire drilling operation while the surface equipment is disassembled to install a new banjo box.
Accordingly, the need exists for a banjo box apparatus particularly adapted to redirecting the high-velocity vertical flow of debris-laden fluids out of a geothermal wellbore, especially those encountered during air drilling of geothermal reservoirs, and having a high wear interior surface capable of withstanding impact of fluids and debris throughout an entire drilling operation.